1. Field of the Invention
The present invention relates in general to fuel nature sensors used in a motor vehicle, and more particularly, to fuel nature sensors of a type which can sense the nature of gasoline used as fuel for internal combustion engines of the vehicle. More specifically, the present invention is concerned with a gasoline nature sensing system which can determine whether the gasoline fed to the engine is light gasoline, medium gasoline or heavy gasoline.
2. Description of the Prior Art
Gasoline used as fuel for automotive engines is generally grouped into three, which are light gasoline which contains as major components chain hydrocarbons such as heptane, pentane or the like, heavy gasoline which contains as major components aromatic hydrocarbons such as benzene or the like and medium gasoline which lies between the light gasoline and the heavy gasoline. Due to the nature of the major components contained therein, the light gasoline is easy to evaporate, while the heavy gasoline is hard to evaporate.
Usually, automotive engines are set to use the light gasoline. However, nowadays, for dealing with the energy shortage and the air pollution suppression law, it has been proposed to use as the fuel of the automotive engines the heavy gasoline.
However, if the heavy gasoline is used in an engine which has been set to use the light gasoline, undesired ignition delay tends to occur causing a lean combustion in the engine. This lean combustion brings about not only poor startability of the engine but also poor operability of the same. Furthermore, usage of such heavy gasoline in such engine tends to induce undesired discontinuous combustion in the engine under cruising of the vehicle. The discontinuous combustion causes increase in harmful components in exhaust gas from the engine.
While, if the light gasoline is used in an engine which has been set to use the heavy gasoline, over-rich combustion tends to occur in the engine, which brings about a so-called "smoking" near the ignition plugs.
In order to solve the above-mentioned drawbacks, a system has been proposed by the same applicants, which is disclosed in Japanese Utility Model First Provisional Publication 4-8956. The system embodies the theory that the dielectric constant of gasoline has a closed relationship with the nature of the same. That is, the dielectric constant of heavy gasoline is greater than that of light gasoline.
In the system, a capacitance detector is used for producing an information on the dielectric constant. The capacitance detector comprises spaced electrode plates which are submerged in gasoline. Electric capacitance established between the spaced electrode plates is treated by an oscillating means which generates pulses whose frequency is based on the capacitance. A frequency-voltage converting means converts the frequency to a corresponding voltage. A judging means judges whether the gasoline is the light gasoline or the heavy gasoline by comparing the converted voltage with a reference voltage.
However, some gasoline put on the market nowadays is of a type which contains as additive agents alcohols such as methanol, ethanol, methyltertialbutylether (MTBE) and the like. Due to addition of such agents, the dielectric constant of the gasoline is increased as is understood from the graph of FIG. 9.
Due to its inherent construction, the above-mentioned conventional system can not deal with such gasoline. That is, as is seen from the graph of FIG. 9, the output voltage "V0" from the frequency-voltage converting means may appear in three cases, one being for heavy gasoline without alcohols, one being for medium gasoline with 5% of alcohols and the other being for light gasoline with 10% of alcohols. This means that the above-mentioned conventional system can not detect the nature of the gasoline any longer.